Modern technology has increased the incidence of a wide variety of compounds in the environment. The effect of exposure to these compounds can be predicted to some degree by toxicological evaluation. Respiratory tract exposure to these compounds is less understood than direct ingestion. A special techlology aimed at increasing our knowledge of respiratory tract exposure has evolved.
Various inhalation exposure apparatus have been developed with the object of providing controlled levels of contaminants to animals in order to assess their impact on these animals. Typically, a plurality of animals is placed in a single exposure chamber into which a test atmosphere is discharged. The animals are usually placed in separate holding cages which are often stacked vertically. This type of arrangement is usually referred to as a whole body exposure technique.
A nose-only inhalation exposure technique offers many advantages over a whole-body exposure technique. The exposure of only the nose of the animal to the test material reduces dermal exposure and gastrointestinal exposure due to grooming of contaminated fur. Dermal exposure and gastrointestinal exposure introduces other experimental variables that often mask the effects of pure inhalation, thereby frustrating the understanding of respiratory tract ingestion. Additionally, the nose-only inhalation exposure technique allows the use of a small size chamber for containing the test atmosphere. This reduction in size enables high concentrations of test materials to be achieved with the use of smaller amounts of test materials. These high concentrations are usually mandated by the evaluation protocols established by the various regulatory agencies such as the Environmental Protection Agency. Reduction in size of the chamber contaminated, reduction in the amount of toxic test material used, and reduced pelt contamination allow easier clean-up and greater safety in handling toxic test materials.
A primary design criterion for inhalation exposure systems used in toxicology testing is that the concentration of test material supplied by the exposure plenum be the same as that concentration at the breathing zone of the test animal. This allows the biological effects observed in test animals to be related to actual concentrations measured in the exposure plenum of these systems. Another prime criterion is that the technique used produces the least amount of stress or other conditions which can alter the biological response of the test animal to the toxic test material.
Many attempts have been made to match these design criteria but no particular design has been found to be completely acceptable. A rubber dam or diaphram fitted around the neck of the animal has been tried. This diaphram is ineffective as it invariably leaks and is difficult to achieve a seal, it is uncomfortable thereby increasing animal stress levels, it is generally hard to use, it needs replacement frequently because the construction material is usually a thin pliable rubber, e.g., latex that tears easily and its prolonged exposure to aerosols, especially those containing oils, promotes rotting or other destruction of the latex.
In the better designs, a major problem has been the ability of the test animal to withdraw its nose from the test material, especially a particularly obnoxious test material, and re-breathe previously exhaled material, rather than fresh test atmosphere. Any re-breathing, whether caused by the test animal withdrawing its nose or by prevously exhaled materials contaminating the supply of test atmosphere, tends to skew test results as exposure concentrations of test materials cannot be determined with any degree of reliability and accuracy.
The present invention overcomes the varied problems of the prior art by providing an apparatus and a method for exposing animals to a test atmosphere in such a manner that the animals are exposed to a uniform concentration of test atmosphere containing substantially no exhaled materials.